The manner in which central dopamine (DA)-containing neurons develop axons and establish functional connections with their target cells in the corpus striatum (CS) and frontal cortex (FCx), and the role of such connections in cell survival, are important issues in the developmental neurobiology of these cells. This laboratory has examined these developmental problems using an in vitro, dissociated cell-reaggregation system in which single cell suspensions prepared from specific areas of the embryonic mouse brain are allowed to reaggregate either by themselves or with cells from other specific brain regions. Only when cocultured with cells from target areas such as the CS or FCx do the DA neurons elaborate extensive axonal processes and exhibit enhanced survival. This target cell-dependent effect on DA neuron development is associated with membrane-bound factor(s) of the target cells. An immunological strategy for identifying and characterizing these neuronotrophic factor(s) utilizing local cell lines is described. There are important advantages to using such cell lines derived from these target cells as a source of antigens for generating the specific antibodies directed against the factor(s). Homogeneous immortal cell lines provide practically unlimited starting material, and they are likely to be enriched in these factors compared to the heterogeneous cell population constituting primary CS and FCx cells. Other cell lines derived from fusions of primary DA-containing neurons with a neuroblastoma cell line have produced hybrid cell lines that produce DA in concentrations equivalent to those found in the adult striatum. A further characterization of these DA-containing hybrid cell lines with respect to neurotransmitter synthesis and neurotransmitter receptors is a prelude to the transplantation of such cells into denervated rat CS in an attempt to reverse the behavioral abnormalities associated with dopaminergic denervation, an animal model of Parkison's disease. Finally, the functional organization of DA neurons in relation to GABAergic target cells of the CS will be assessed with pharmacological tools. The ontogeny of target cell D-1 and D-2 receptors allowed to develop, alone, or in the presence of a developing dopaminergic innervation will be examined. These DA neuronal systems are important sites of action for pharmacologic agents, and these experiments should yield further information on their normal development as a function of the cellular and chemical environment in which such development occurs.